Blender Character Modeling and Gravity
Modeling a character with its arms by its side can cause issues at two stages. While modeling or sculpting having two disconnected parts together can make selecting and moving each part separately harder. Then while rigging the character, it can make it harder to specify which parts follow each bone.

Whether you model a character with arms straight out or half way down is mostly a matter of taste, each helps the modeling/rigging process. Some people suggest the more relaxed pose is better as the mesh is closer to the normal position so there is less stretching around the shoulder.

As for drawing a modeling sheet, draw them in the position to be modeled. It's not uncommon to have the side view of a character with the arms moved to the side to keep the drawing clearer to read. You may be interested in this CGCookie tutorial on drawing a modeling sheet